This application claims the priority of Japanese Patent Application No. 2000-037881 filed on Feb. 16, 2000, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a high-ratio zoom lens mountable to a handy-type TV camera; and, in particular, to a zoom lens using a three-element achromatic lens as a variator.
2. Description of the Prior Art
In recent years, four-group type zoom lenses having a high power variation have come into use as being mounted to handy-type TV cameras.
For making the TV cameras smaller, demands for reducing the size of such four-group type zoom lenses have been increasing year by year. If those having a high power variation are to be made smaller, then their various kinds of aberration become greater, against which measures are necessary.
Among them, it is not always easy to correct chromatic aberration occurring upon zooming or focusing, such as lateral chromatic aberration in particular. For correcting such chromatic aberration, so-called counter achromatization technique has been known, in which, when the variator has a negative refracting power, the dispersion of a positive lens among lenses constituting the variator is made greater, whereas the dispersion of a negative lens among these lenses is made smaller. A cemented lens of such a combination may be used so as to correct the lateral chromatic aberration.
As the achromatic lens, those comprising three elements such as one shown in FIG. 14 in which another lens is combined with the cemented lens in order to increase the number of lens surfaces in view of the freedom of design have conventionally been known.
When such a three-element achromatic lens is used, however, the total length of lens system further increases, which contradicts the demand for compactness. This point will now be explained specifically with reference to FIG. 13.
FIG. 13 schematically shows a principal ray of a luminous flux corresponding to a marginal portion of a picture when a second lens group, which is a variator contributing to power variation, is positioned at a wide-angle end (closest to the object). The solid line and dotted line indicate the principal ray in the case where the achromatic lens of the second lens group is constituted by two elements and by three elements, respectively. Here, xe2x80x9c2xe2x80x9d is a stop, which is configured such that a third lens group G3, i.e., a compensator, is positioned upstream thereof. According to FIG. 13, the lens thickness in the case of a second lens group Gxe2x80x22 whose achromatic lens is constituted by three elements becomes greater than that in a second lens group G2 whose achromatic lens is constituted by two elements, whereby the lens diameter of a first lens group Gxe2x80x21 positioned upstream the second lens group Gxe2x80x22 becomes greater than that of a first lens group G1 positioned upstream the second lens group G2.
In view of such circumstances, it is an object of the present invention to provide an achromatic lens exhibiting a counter achromatization effect, which can satisfy the demand for compactness while fully securing the freedom of design by increasing the number of lens surfaces; and a zoom lens using the same.
The zoom lens in accordance with the present invention comprises, successively from an object side, a first lens group, fixed upon zooming, having a positive refracting power; a second lens group, movable upon zooming, having a negative refracting power; a third lens group, having a negative refracting power, for correcting a fluctuation of an image surface upon zooming; and a fourth lens group, fixed upon zooming, having a positive refracting power;
wherein the second lens group comprises, successively from the object side,
two meniscus lenses each having a negative refracting power with a convex surface directed onto the object side; and
An achromatic lens comprising, successively from the object side, a cemented lens having a negative refracting power as a whole, and a lens having a positive refracting power with a convex surface directed onto the object side; the cemented lens comprising, successively from the object side, a meniscus lens or plane-convex lens having a positive refracting power with a concave surface or plane directed onto the object side, and a lens having a negative refracting power.
Preferably, the first lens group comprises, successively from the object side, a first-a lens group, fixed upon a focusing operation, having a positive refracting power; and a first-b lens group, movable to the object side upon a focusing operation from an object at infinity into an object at a shorter distance, having a positive refracting power.
Preferably, the zoom lens of the present invention satisfies both of the following conditional expressions (1) and (2):
xcexd23 less than 25.0xe2x80x83xe2x80x83(1) 
xcexd25 less than 25.0xe2x80x83xe2x80x83(2)
where
xcexd23 is the Abbe number xcexdd of the lens having a positive refracting power constituting the cemented lens in the second lens group; and
xcexd25 is the Abbe number xcexdd of the lens having a positive refracting power with a convex surface directed onto the object side constituting the cemented lens in the second lens group.
Preferably, the zoom lens of the present invention satisfies the following conditional expression (3):
0.30 less than f2/f2a less than 0.60xe2x80x83xe2x80x83(3)
where
f2 is the focal length of the second lens group; and
f2a is the focal length of the cemented lens in the second lens group.
Preferably, the zoom lens of the present invention satisfies the following conditional expression (4):
|N23xe2x88x92N24| less than 0.02xe2x80x83xe2x80x83(4)
where
N23 is the refractive index Ne of the lens having a positive refracting power constituting the cemented lens in the second lens group; and
N24 is the refractive index Ne of the lens having a negative refracting power constituting the cemented lens in the second lens group.
The achromatic lens in accordance with the present invention comprises, successively from the object side, a cemented lens having a negative refracting power as a whole, and a lens having a positive refracting power with a convex surface directed onto the object side; the cemented lens comprising, successively from the object side, a meniscus lens or a plane-convex lens having a positive refracting power with a concave surface or plane directed onto the object side, and a lens having a negative refracting power.
In this case, a lens group including the achromatic lens may be movable upon zooming.